Process and apparatus for burning lime



June 11, 1929. c. J. WRIGHT PROCESS AND APPARATUS FOR-BURNING LIME 3Sheets-Sheet Filed Aug. l5 1923 s sheets-sheet 2 c. J. WRIGHT PROCESSAND APPARATUS FOR BURNING LIME Filed Aug. l5

June H, 1929.

June ll, 1929. C, 1 WRIGHT L'ZG'Z PROCESS AND APPARATUS FOR BURNING LIMEFiled Aug. 15, 1925 3 SheetSv-Sheel', 5

Patented June" Il, 1929.

UNITED STATES cARL J. WRIGHT, O E WEsT NEW BRIGHTON, NEW YORK, AssIGNoR,:BY IIEsNE AssIGNMENTs, To cOMBUsTION UTILITIES CORPORATION, OE NEWYORK, N. Y.,A

CORPORATION OE MAINE.

PROcEss AND ArRARATUs EOR BURNING LIME.

Apucation med August 1 5, 1923. serial No. 657,492.

-Thisfinvention relates to the manufacture of lime and carbonio acid gasfrom carbonates in a continuous manner.

A large part of the lime made' in this coun- 5 try is calcined in a kilneither in direct contact with the fuel or with the fuel gases,

. so that. the ultimate products are moreor less impure. In such aprocess the gaseous products of combustion ofthe fuel. such as nitrogenof the air used for combustion, sulphur dioxide 'and ammonia becomemixed with the carbonicacid'gas, giving a gaseous product With a lowcarbonio acid content, which is not eflicient for use in soda ashmanufac- 15Ature, and is too `impure for other purposes. In addition togiving impure carbonio acid gas, this method of burning lime isexpensive Y and inetlicient, and is diiicult to-control, often'resulting in underburning or overburning a -part of the lime. Moreover,the solid and liquid products of decomposition of the fuel,

such as ash, tar and clinkers arewithdrawn in considerable-amount mixedWith the lime,

i) r resulting in an unclean lime 'with 4poor slacking qualities.Various attempts have been made to :overcome these diiliculties byemploying other heating means, 'in particular icurrents of lsuperheatedair or superheated steam, but' it isnot known that anyof these ao.attemptshave resultedin aprocess -Which 'could be adopted in .generalcommercial practice.

The'primary object of tlie present inven.

tion iis to provide a process for burning lime by which lime may beeici'ently and economi# cally produced and comparatively pure carbonioacid gas may be obtained While burning lime.'

'Another object ofthe present invention isl l r 40 `to provide a'process for burning lime by which a minimum 'amount of heat is lost dueto the removal of highly heated products from theapparatus in which theprocess is carried out. v-

Another object of the present invention is to provide a process forburning. lime by which the temperature'will be accurately controlled sothat the lime will :not be under or over burned.

' Another object ofthe present invention is to provide acommerciallypracticable -process for burning. lime by Which the decompositionltemperature of the lime is obtained solely by heat transfer from achemically inert gaseous mixture.

A further'object of the invention is to provide an apparatus in whichlime may be eificiently burned and the above process carr1ed out. Withthese and other objects in viewthe invention consists in the improvedprocess and apparatus for burning lime hereinafter described andclaimed.

The various features of the invention are illustrated in theaccompanying drawings,

in which s Fig. 1 is a more or' less diagrammaticalliliv view in sideelevation with parts in-section'.

of a lime burning kiln .and 'the auxiliary gas treating apparatusembodying the preferred form ofthe invention.

Fig. 2 is a view in plan partly in section of the apparatus shown inFig. 1. Fig. 3 is an enlarged horizontal cross `sectional view of thelower portion of the kiln showing in plan the mechanism for supf portinga charge column of limestone in the kiln and for removing charge thefoot of the column.

Fig. 4c is an enlarged 'view in Kvertical section of the chargesupporting and discharge mechanism' shown in plan in Fig. 3, taken ontheline 1 -4 of Fig. 3.

The process and apparatus ofthe present invention is so designed thatlime may be economically vand continuously4 burned and substantiallypure lime and carbonio acidgas apparatus any gaseous medium preferredmay be superh'eated to the temperature desired vfor carrying on achemical reaction. According to the preferred form of this invention amix-v ture of carbonio acid gas and steam is superheated to:1t-temperature of about 1900o F. tov

material rom 'may be obtained. By the use of an improved that the limewill not be under or over burned,-

and the resulting' gaseous mixture composed through a condenser toremovethe. steam, v

leaving substantially pure carbon dioxidev of carbon dioxide and' steamcan run- Y ratus.

' production of lime and carbonio acid gas at a considerably lower costthan has heretofore been realized in commercial practice. n w

To carry out the improved process inthe apparatus illustrated in the.drawings,lime stone is passed in -a column downwardly through a shaftkiln 10, Fig. 1. The limestone is dropped from a bin 12 into a charginghopper 14 located at the top of the kiln, by opening a gate 16 on thebottom of the bin. The gate 16 is then closed and a gate 18 at thebottom of the charging hopper is opened, permitting the limestone in thecharging hopper to flow into the top of the kiln. The charging operationis an intermittent one, and the gates 16 and 18 are so operated thatwhile one is open the other will always be closed,

in order to prevent the escape of gases `from the kiln into the bin. Thelimestone is sup#A ported in the furnace in a '.ertical column, and iscontinuously advanced downwardly through the column, In its passagethrough the furnace the column of lime is passed through three distinctzones,namely, a heating zone 20, in the upper part of the shaft, a hotmid zone 22, and a lime cooling zone 24 in the lower part of the shaft.The limestone is decomposed in the mid zone 22 of the kiln, at atemperature of from 1400o to 2000o F., into lime and carbonio acid gas,according to the following reaction:

' The gate 26 is shown as a revolving trunl Y has been previously heatedby the heat of combustion of producer gases whichl have been generatedin some form of Stoker.

The Stoker indicated as 28 in Fig. 2 may he yan ordinary coal furnace, agas producer, a

. gas burnerfor an oil burner.

' hot conduits In this Stoker'Y the cheapest type of fuelcan be burned,and the products of combustion are sentthrough 0 or 32, to one of a pairof regenerators or' reheaters 34 or 36, as shown. The circulati ,n ofthe hot gases is controlled by hot valves x38 and 40 which areespecially designed to withstand high temperatures,

and exhaus'ters 62 or 64.

and preferably, arrangement made so that ,the pressure on the oppositesides of the valves issubstantially the same, so that there is a mmlmumleakage. When one regenerator 1s `being heated by the hot gases, theother regenerator which has previously been heated.

is used vfor preheating carbonio vacid as and steam. The hot, furnacegases a ter passing through either of the regenerators.

are drawn through conduits 42 or 44, provided with hot valves 46 or 48,into feed water heaters, 50 and 52, and from the feed water heaters theyare 'exhausted to the atmosphere throughstack pipes 54, 56 or 58, 60,The feed water heaters have water pipeconnectionsl v66 and 68 with asteam boiler 7 0, and a part of the hot gases from the stoker are drawnthrough a conduit 72 to be utilized in generating steam in the boiler,at substantially 212o F. and atmospheric pressure. The only pressurebuilt up in the boiler is that required for positively controlling thecirculation 'of steam. v

Steam from the boiler is admitted into the lower endof the kiln bylmeans of pipes 74 connected to ahorizontal coil 76 placed a shortdistance above the discharge gate., said coil being perforated'in such away that the steam will be evenlydistributed throughout the body of limein the-lower part of the kiln.

To support the column of lime and to insure an equalized downwardmovement of the charge throughout the kiln, as well as to aid in evenlydistributing steam throughout the body of the charge, a grate is set inthe lower part of the shaft just above the steam -coil 7 6j ln itspreferred form this grate may consist of a` series of hoppers 78, thatis six peripheral hoppers andra central hopper, formed respectively bythe opposed sloping sides of six uniformly spaced and peripherallydisposed, stationary tent shaped supporting members 79 and a centrallydisposed ring shaped member 81 of conical cross section, connectedtherewith, with discharge openings80 in the bottom of each peripheralhopper through which the lime passes from the bottom of the column ontoaseries of movable platforms 82. The platforms 82 are given areciprocating motion by means of water operated engines 84, to dischargethe lime from the platforms into a hopper 86, forming the` bottom oftheshell. A more complete description of this discharge mechanism willbe found in the British patent to Henry O. Loebell, N o. 216,488,6May23, 1923. The amount of steam admitted into the bottom of the chargethrough the perforated coill 76 is controlled so as to cool the lime atthe discharge gate to a comparatively low temperature, that is, to aslow a temperature as is consistent with' the production of quick lime asdistinguished from slack lime at the base of the column, or in ptherwords to a temperature above the dissociation temperature of calciumhydrate. 'In burning lime'-a stone to recover lime and carbonio acidgas, the reaction which takes place is reversible in cases where anexcess of carbonio acidgas may come in contact with the lime. Theintroduction of steam into the bottom of the charge 0drives out anycarbonio acid gas which might be carried down with the lime,.pre ventsany reversal of the, primary reaction and insures the complete burningof the lime. As the steam rises toward the high tempera- 'ture zone ittakes up heat from the descendused in the present process for heatingthe mid zone of the descending body of limestone to a temperature suchthat complete decomposition takes place, consists of inert gases, and ispreferably composed of about CO2 and 30% steam by Weight. The mixture ofcarbonio acid gas and steam is forced by a blower through the hot brick.Work 88 of one of .the pair of regenerators or preheaters shown, whilethe otherregeneratoris being heated by recovering. the heat ofcombustion of furnace gas therein. When one regenerator has been cooledto the minimum Working temperature which' is desirable, by passingcarbonio acid gas and steam through it, the

balance. .After passing through the oftake the mixture of gases isdivided at the T 104, a partbeing reoirculated through the blowere102and pipes 106 or 108 through the regenerators and kiln to maintain thehigh tem era- 70,

ture in the mid portion of the body of ime, While the remainder of thegas mixture is run through a tubular condenser 110, in which the steamis condensed and from which the practically pure carbonio acid gas maybe passed throughs a fan .116 and discharge pipe 117 to soda ashabsorption towers or to other use in the arts. water from the tubularcondenser may be led )to the feed water heaters by pipes 112 and .80

114, and from there to the boiler, vif desired. The amount of gaspassingthe otake which is recirculated, and the amount which isWithdrawn through the condenser, is gov-v erned by the speed of theexhauster 102,*and

the speed of the fan 1,16 in the condenser circuit. Y

JThe shaft furnace 10 is arranged vto support the fuel in a verticalcolumn in suc a manner that the limestone may be1 continuouslyad- 90having a top cover. consisting of the charging 'hopper 14 and the bin12,.With'theintermit. ltently operating charging valves 16 and 18, and aAlime discharging hopper 86 at its circulation of the steam and CO2'mixture is lower end in which the discharge gate 26 is stopped andthenstarted in the other regen.

erator, which was being heated during the time when the firstregenerator was used for heating the mixture `of steam and CO2. In-

- into a, passage 98 surrounding the fuel column, by means of which itis evenly distributed throughout the cross-section of the column. `Herethe mixture 'of blast gases is joined by the steam which was introducedat the bottom of thecolumn of lime, and by the carbonio acid gasproduced in the decomposition of the limestone. As this body of mixedgases asoends toward the top ofthe kiln it is cooled by a heat transferwith the Vbody ofl descending limestone in the heating zone 20 so thatthe limestone is gradually heated as it approaches the high temperaturezone 22 of the kiln, and the mixed gases as lthey approachv an offta'kepipe 100 at the top v of the kiln are gradually cooled to a located; Therefractory lining 120 is pref- 100 erably positioned in all the inner.surface-of the shell which comes into contact with the heated limestone.The shell and lining are both so' constructed as to graduallyinoreasethe cross-sectional area of -the column of lime- 195 stone asitadvancesto the mid zone of the furnace, in order that it may expandinto a larger spaceto be in an openv condition to assist the cross blastinI penetrating through the column.`

'.l`he.rei.ractoryl lining is constructed so as to 110 form a flat firearch 122 immediately above the blast necks 90 and 92thus forning the.passage 98 between the descending body of the( charge and the liningimmediately beneath the iat arches around the whole circumference of theshaft, through which the hot gases f will distribute themselvesequallythroughout the central section of the descending charge.

theqross sectional area-of the column of lime 125 as it descends fromthe hot mid zone toward the discharge, so that the calcined lime, whichhas lost in volume during thel burning process, is compressed gradually'as vit cools, bythe weight of the descending'column, and'this 130 Thecondensed steamand the These Hat arches confine the cross blast ofsuperheated gases to the zone 2.2 and prevent a direct application ofthe hot'blast compression of the burnt lime assists the cooling andcleansing action of the steam ris ing from the coil 76.

The regenerators or preheaters 34 and 86 for preheating'the inert blastgases and for absorbing the heat of the stoker lgases preferably consistof cylindrical gas-tight shells which have refractory linings 124covering their innersurface. The heat recuperation of the gases in thei'egenerators is'efl'ected mainly by means of a series of checker bricks88 which are supported on grates positioned near the bottom of theshells and extend to the level of the iiues 30, 32, 90 and 92,. Hotvalves are positioned in the Stoker conduit, the blast neck, the gascircuit line fromithe blower, and the combustion gas conduit betweenthe-regenerator and feed water heater,

fractory material, which is connected to a head, and is moved throughslots in the conduit in which the valveA is located, by hydraulic power.

Incase no supply ofcarbonic acid gas or of mixed carbonic acid gas andsteam is readily available at the time the present lime burning processisfirst put into operation after a shutdown, the heat requiredforputtiug the process into the continuous normal operation abovedescribed is'supplied in the following` manner: After introducing acharge of limestone into the kiln a high degree of preheat is firstimparted to the refractory linings of preheaters 34 and 35, preferablyby the combustion ofgas'eousfuel from Stoker 28 in heat transferring Yrelationship therewith, and steam is then conducted from boiler -70through branch pipe connections 126 and conduits 108 preferably into thebase of both preheaters.' In passing through the heated linings of thepreheaters a hivh degree of superheat, about 1900o to 2000o isini-parted to .the steam and this superheated steam is then sent throughthe charge of limestone to raise the temperature off the charge` tothatpoint at whichdecomposition into calcium 'oxideland ocarbonic acidgas begins in the blast zone. After thus initiating the lime burningoperation this initial 'supply 0f superheatcd steam is preferably .cutpli' by closing valves 128 in the pipesi126 and thereafter the heatsupply :for maintainingAthe hme burnmg operation. iS carried to "the.charge by the circuitof mixed Steam and 'carbonic acid gas withdrawnfrom the top of `the kiln, as already described.

y.YF-rom the foregoing description it is evident that a very eiicientrecover 'of heat is obtained from the combustion o the cheapest type offuel in the Stoker, and also from the solid and gaseous productsof thedecomposition of limestone in the furnace. This efficient heat recoveryand the fact that a cheap fuel may beutil'ized, cuts the cost of fuel toless than a half of that necessary in present day lime burningoperations. At the same time practically pure products are produced; andthe control of 'the operation is so closely governed as to insure thatthe lime will always be buried to just the degree desired. p

By using a mixture of steam with carbonic acid gas, or other chemical] yinert gases Such as air or nitrogen the tension of the expelled carbonicacid gases is reduced, due to partial pressures, and this fact permitsthe liberation of the carbonic acid gas at somewhat lower temperaturesand in a shorter period of time than is obtained by using a single gasas heating medium.

The preferred form of the invention having been thus described what isclaimed as new is:

1. A process of burning limestone comprising, continuously passinglimestone downwardly\in a column through a shaft kiln, effectingcontinuous thermal 'decomposition .of the limestone in the mid-portionof the column by contactingit`with a stream of preheated inert Ygas,coolin r the lime product of said decomposition in the lower portion ofthe column by contacting it with a counter-flow stream of saturatedsteam., and continuously withdrawing cool lime from the foot of saidcolumn and a cool mixture of steam and carbon dioxide from the'top ofsaid column.

ion

2. A process of burning limestone comi prising, continuously passinglimestone downwardly in ,a column through a shaft f kiln, effectingcontinuous thermal decomposition of the limestonein the mid-portion ofthe column by contactingit with a stream l of preheated insert gas,cooling the lime product of said decomposition in the lower portion ofthe column by contacting it with a counter-How stream of saturatedsteam, continuously' withdrawing lime from the foot of said column and acool mixt-ure of steam and carbon dioxide 'formed' from the top ofsaidcolumn, and controlling the temperature and volume of Said inert gasstream and the volume of said saturated steam cooling medium to preventunder or over burn-- ing of lune 1n said decomposition zone.

. 3. `A`process of burning.. limestone com- 4,25.

prising, continuously advancing limestone downwardly in a column'througha shaft kiln, eiiecting'pcontintious thermal decomposition of saidlimestone by contacting it with a superheated mixture of steam andcarbon\dioxide introduced circumferentially into' the mid-portion ofsaid column and passed upwardly therethrough,'cooling the lime productof said decomposltion by contacting it with steam introduced at the baseof. said column and passed upwardly therethrough, and controlling thevolume of saturated steam thus introduced to maintain the temperature ofthe lime at the foot of said column only slightly above the dissociationtemperature of calcium hydrate.

4. A process `of burning limestone comprising, continuously advancinglimestonev downwardly in a ,column through a shaft kiln, effectingcontinuous thermal decomposition of the limestone in the mid-portion ofsaid column by direct heat transfer with a` .superheated mixture ofsteam and carbon wardly in a column through .a shaft kiln,

effecting continuous thermal decomposition of the limestone in themid-portion of said column by direct heat transfer with a superheatedmixture of steam and carbon dioxide, regenerating a substantial part of.the sensible heat of the lime formed by heat transfer with saturatedsteam introduced at the foot of the' column and passed upwardlytherethrough, regenerating a substantial part of the sensible heat ofthe carbon dioxide formed bycontacting it with the limestone in the up.-per part of the column, withdrawing cooled lime from the foot of thecolumn and a cool carbon dioxide-steam mixture from the top of n thecolumn, and superheating part of the car-` bon dioxide-steam mixturethuswithdrawn and reintroducing it into the mid-portionof said column. j

6. A process of burninglimestone comprising, continuously advancinglimestone downwardly in a column through a shaft kiln, effectingcontinuous thermal degomposition of the limestone in the mid-portn ofsaid column by direct'heat transfer with a superheated mixture of steamand carbon dioxide passedupwardly therethrough, regenerating asubstantialpart of the sensible heat of the lime formed by heat transferwith steam introduced at the foot of the column and passed. upwardlytherethrough, regenerating a substantial part of the sensible heat ofthe cary bon dioxide'formed by contacting it with the limestone in theupper part ot the column,

continuously withdrawing cool lime at a con- `fecting continuous thermaldecomposition of 190 trolled rate from the foot of the column and acooly carbon dioxide-steam mixture from the top .of the column, andcontinuously superheating a part of the carbon dioxide-steam mixturethus withdrawn and recycling it `through the upper half only qfsaidcolumn.

7. A process ofvburning limestone comprising,- continuously passinglimestone. downwardly in a column through a 4shaft kiln, effectingcontinuous thermal decomposition of the limestone in the mid-portion ofthe column by contacting it with a counter-How mixture of super-heatedsteam and-carbon dioxide, preheating thejlimestone in the upper part ofthe column prior to such decomposition by contacting it with the carbondioxide product of said decomposition, su-

perheating steam used in eectlngv such decomposition by contacting itwith the lime product ofsaid decomposition in the lower part of thecolumn, continuously withdrawing cool lime at a controlled rate from thefoot of the column and a cool steam-carbon dioxide mixture from the topof the column, superheatinga part of the steam-carbon dioxide mixturethus withdrawn and recycling it through the upper half of the column,and supplementing the volume of said recycled mixture with -the steamsuperheated' in the lower part of ,the column and with .the

carbon dioxide product of said decomposition.

8. A process of burning limestone comprising, continuously Apassing`limestone downwardly in a column through a shaft kiln, ef-

fecting continuous thermal decomposition of the limestone in themid-portion of the column by contacting it with a counterow mix-- tureof superheated steam and carbony dioxlde, preheating the limestone inthe upper "part of the column prior to such decomposition by contactingit with the carbon dioxide product ofsaid decomposition, superheatingsteam used in effecting-such decomposition by contacting it with thelimeproduct; of

ics

said decompositin in the lower part 'of the' column, continuously,Awithdrawing l' e at a controlled rate from thefoot of .thecolumn and asteam-carbon dioxide mixture from the top of `thecolumn, superheatingpart of the steam-carbon dioxide mixture thu s withdraw-n and recyclingit ythrough the'upper half ofthe column, supplementing the Volume .ofsaid rec cled mixture' with the steam superheated in the lower part ofthe column and with the. carbon dioxide product of said decomposition,and se arating steam from the `part of the car on dioxide-steam' mixturenot thusmecycled and.

collectingethe carbon dioxide resulting' from said separation.

y9. Aproce'ss of burning'limestone compris-i ing, continuously passinglimestone down-z wardly-in a coluinn through a shaft kiln, ef-

iccvv v ltheflimesftonein the mid-portion of the column by contacting itwith a counter-How mixture o superheated steam and carbon diox- 1de,preheatmg the limestone in the upper part of the columnprior to suchdecomposi-V tion by contacting it with a counter-current stream ofcarbon dioxide product of said decomposition, 'superheating steam usedinv effecting such decomposition by contacting it with the lime productof said decomposition in the lower-part of the column, continuouslywithdrawing lime 'at a controlled rate from the foot of the column and asteam-carbon dioxide mixture from the top of the column, superheatingpart of the steam-carbon dioxide mixture thus withdrawn and recycling itthrough the upper half of the column, supplementing the volumeoisaidrecycled mixture with the steam superheated in the lower part of thecolumn and with the carbon dioxide roduct of said decomposition, andcontrol ing the volume of saturated steam introduced into the foot ofthe column to maintain the composition of the steam-carbon dioxidemixture removed from the top of the column substantially constant.

l0. A process of burning limestone comprising, continuously advancinglimestone downwardly in a column through a shaft kiln, heating thelimestone in the mid-portion of the column to a temperature suflicientlyhigh to decomposeI it by contacting it with a superheated mixture ofsteam and carbon dioxide gas, vcooling the lime formed as it advancestoward the foot of the column by contacting withy steam, graduallyexpanding the limestone as it advances toward the mid-portion of thecolumn, gradually compressing the lime formed as it advances toward thefoot of the column, and withdrawing comparatively cool lime from thefoot of the column and comparatively cool carbonic acid gas and steamfrom the top of the column.

, foot of the column, withdrawing comparatively cool lime' from the footof the column and comparatively cool carbonic acid' gas and steam fromthe top of the column, and controlling the volume of steam intrcfduced.at the foot of the column andthe 'temperature and volume of saidsuperheated carbon dioxide-steam mixture to prevent underoroverburning'of the lime and to maintain the comf position of the carbondioxide-steam mixture removed from the top of the column substantiallyconstant.I y

12. A process of burninglimestone comprising, continuously advancinglimestonev downwardly in a column through a shaft kiln, superheating amixture of carbonic l acid gas and steam, decomposing said limestone byintroducing said'superheated carbonic acid gas-steam mixtureinto themidportion of the column and passingit upwardly in contact with thelimestone in the upper half ofdthe column, supplementing the heatingeiect of said superheated carbonicacidsteam mixture with an additionalsupply of superheated steam developed inthe lower portion of the columnatthe expense of heat regenerated from the lime product of saiddecomposition, withdrawing lime from the. foot of said column and acarbonic acidgassteam mixture from the top of said column, maintainingthe composition of the carbonic acid gas-steammixture thus withdrawnsub-4 stantially constant at about 30% steam and carbonic acid gas byweight, and superheating and recycling a portion said carbonic acidgas-steam mixture tlrough 4the upper half of said limestone column.

13.`A process of burning limestone comprising, continuously passinglimestone downwardly in a column through'a shaft kiln, effectingcontinuous thermal decomposition of the limestone in the mid-portion otthe column by contacting it with a super'- -heatedfmixture of steam andcarbonic acid (gas, continuously circulating superheated steam andcarbonic Iacid gas ,through the upper half of said column, regeneratingthe sensible heat of the carbonic acid gas formed by heat transfer withlimestone in the upper portion of the column, regenerating the sensibleheat ofthe burnt lime by heat transfer with steam in the lower portionof the column, returning the heat thus regenerated' to the mid-portionof the columnin the form of preheated limestone and superheated steam,respectively,v and continuously with-l drawing the lime from the foot ofthe column;

and a carbonic acid gas-steam mixture from" p the top of the column; I,

v14. lA process for burning limestone com-i prising passing a column oflimestone through a shaft kiln, decomposing said limestone byintroducing a superheated gaseous mixture comprising carbonic acid gasand steam into the mid portion of said column, introducing saturatedsteam into the bottom ofthe column in an amount such that the gasmixture. leaving the top of the column will contain approximately 30%steam and 70% CO2 by weight, superheating a part of the gas mixturedrawn from the top of the shaft by'passing it-through a hot preheater,and returning said superheated gas mixture to the shaft. L

15.4 A process for :burning limestone comprisin passing acolumn oflimestone through a sha t kiln, decomposing the limestone into lime andcarbonio acid gas by direct contact with a counter-current circuit ofsuperheated steam and carbonio acid gas during its p assage through thekiln, withdrawing carbonio acld gas and 4steam from the top of thecolumn, and regulating the rate at which limestone is passed through thekiln and the volume and composition'of the gas circuit passedcountercurrently therethrough to maintain the temperature of the mixtureof carbonio acid gas and steam leaving the top of the column in theneighborhood of 500 F.

16. In lime burning apparatus, an upright shaft kiln, a gas sealedcharging device 'at the top of the shaft, a gas sealed dischargingdevice at the foot of the shaft, means mounted in the lower portion ofthe shaft for supporting a charge column therein, said means havingcombined therewith mechanism for 'rc- Amoving charge material at acontrolled rate from the foot of the column, a gas oftake at the top 0fthe-shaft, a gas intake in themidportion of the shaft, means forintroducing steam at a controlled rate into the lower portion of theshaft, a gas preheater, a blower, a gas -connection between said offtakeanc the intake sideof said blower, a valved connection between thedischarge side of said blower and said preheater, and avalved gasconnection between said preheater and the gas intake in said shaft.

17. An apparatus for burning lime comprising anupright shaft, adischarging device at` the top of the shaft, a discharging',J device atthe bottom of the shaft, a circumferential enlargement and a gas inletin the mid portion of the shaft, agas outlet in the top portion of theshaft, a preheater, means for circulating gas from the gas outletthrough the preheater and gas inlet back into the midportion of theshaft, and means for introducing aheating gas into the preheater.

18. An apparatus for burning lime comprising a shaft kiln, a chargingdevice for charging limestone into the top of the shaft, a dischargingdevice for discharging lime from the bottom of the shaft, means forcontrolling the movement of lime through the shaft, a gas oftake at thetop of the shaft, atpair of preheaters connected to gas inlets inthe midportion ofthe shaft, means forI heating said preheaters, a blower, a gascon- .nectionbetween the intake of said blower lpreheater gasconnections.

@fill-9. An appparatus forburning lime com-f prising an uprightshaftkiln, means for introducing limestone into the upper portion of theshaft, means for removing lime from the bottom oT the shaft, a gasoiftakeponnected with e top of the shaft, a pa1r of gas inlets connectedwith the central portion of the shaft, a preheaterconnected with each Ainlet and with the gas otake, means to introduce a heating gas into thepreheaters, means for introducing amixture ofinert gases into thepreheaters to be heated, means 'for circulating inert gasesfrom-preheaters through the gas inlets and upper portion of thel shaft,and meansv for cutting of'one preheater from the shaft when the otherpreheateris used to superheat the mixture of v .the bottom of the shaft,a gas oftake at the top of the shaft, a pair of preheaters 0011-' nectedto gas inlets in the mid portion ofthe shaft, means forcirculating gasfrom the Oiftake through the preheaters, gas inlets and upper portion ofthe shaft, a boiler foi` generating steam, feed water heaters, means forsupplying heat to the preheaters, boilerand feed water heaters, waterconnections between said vboiler and heaters, and steam lcg'imectionsbetween said boiler and shaft 2'1. An apparatus for burninglime'comprising an upright shaft kiln, means for charging limestone intothe top of the shaft, means for discharging lime from the bottom of theshaft, means connected to a gas inlet in the mideportion of the shaftfor preheating a mixture of gases before its introduction into `the midportion of the shaft, means for introducing steam linto the bottom ofthe shaft, a gas offtake at the top of thecshaft, a

blower, a condenser, and connectionsl for ref.'

circulating a portion of the gases drawn from .blower and condenser.

22. An apparatus forburning lime com ,95 mel les` the top ofthe shaftthrough the blower and prising an upright shaft, a charging device" atthe top of the shaflt,`a gasoftake at 'the top of the shaft, preheatersconnected to the gasvofftake and to'blast inlets in the centralportionof the shaft, Aa steam distributing coil? in the bottom of theshaft, a steam boiler, a condenser connected with the gas oiftake, feedwater heaters connected to the preheaters,

and connections for leading' water from the. condenser through the' feedwater heatersfto the boiler, and connections for leading steam from theboiler to the coil in the bottom of the shaft and to the preheaters.

23. In an apparatus fonburninglime, an" 4 upright shaftkiln, a chargingdevicel at the" top of the shaft, a discharging device at the foot ofthe shaft, means mounted in the lower portion of the shaft forsupporting a charge' column in the sha-ft and for controlling thethrough said hot gas introducing means and movement of charge materialthrough the /the upperpart of the shaft, said shaft having 10 shaft, agasofl'talre at the top of the shaft, its largest interior crosslsectional dimensions means for introducing hot gas into thek midf at itsmidzone andgraduallydecreased cross '5 portion-ofi the shaft, 1neans forintroducing sectional dimensions toward its top and botsteamfat' acontrolled rate into the lower portom extremities. v tion the shaft, andmeans for circulating In testimony vwhereof I aiix my signature. gasremoved through said offtake hack CARL J. WRIGHT.

